Permalink : http://hdl.handle.net/2115/38445

KEYWORDS : Magnetic susceptibility;Rock magnetism;Curie temperature;Setana Formation;Hokkaido

Rockmagnetic investigations were conducted on sediments constituting the upper part of the Setana Formation and the Chiraigawa Formation, of Plio-Pleistocene age, exposed along the Soebetsu River in the vicinity of Kuromatsunai town, Hokkaido. Average mass-specific low-field magnetic susceptibility (χ) had the lowest value (15.2 x 10^[-8] m^3kg^[-1]) at one site in the Kuromatsunai Formation, but higher values represented by wide ranges of (26.1-132.2) x 10^[-8] m^3kg^[-1] and (135.6-870.1) x 10^[-8] m^3kg^[-1] characterized the Chiraigawa and Setana Formations, respectively. Magnetomineralogical analyses by isothermal remanence (IRM) acquisition, Curie temperature determinations, and electron microscopy aided with energy dispersive X-ray analysis suggest the presence of several types of magnetic minerals: (i) one of a soft coercivity phase, probably a Ti-poor magnetite, with a Curie temperature of ca. 535-555℃, occurring throughout the section; (ii) one of intermediate coercivity, relatively rich in Ti (ca. 10% by weight), with a Curie temperature of ca. 460-475℃, restricted to two relatively thin layers of the Chiraigawa Formation, each several tens of cm thick; and (iii) a hard-coercivity hematite-like and/or a very-hard-coercivity goethite-like phase contributing up to 0.5-5% of the saturation IRM acquired at 2.5 T. Combined use of susceptibility and several ratios or differences derived from IRM acquisition data, bi-plots of IRM vs. χ, and magnetomineralogical inferences were used to discriminate rockmagnetic zones within the Soebetsu River Section. Similar rockmagnetic analyses have proven to be effecive for mapping as well as for lithostratigraphical correlations of geological sections. Combined use of these rockmagnetic zones with other environment/climate proxy data may prove effective in paleoenvironmental reconstruction.